Known in the art is an internal combustion engine which is provided with a variable compression ratio mechanism which can change the combustion chamber volume at top dead center to change the mechanical compression ratio. In such an internal combustion engine, sometimes the amount of intake air is controlled by changing the closing timing of the intake valve in the compression stroke. In this case, it has been proposed to calculate the amount of intake air based on the closing timing of the intake valve and the intake air pressure (see PLT 1).